Perfumery is a craft that is hard to demystify. "It's not about what is mixing in front of you," says perfumer Ruth Mastenbroek, "it's what is going on in your head."

Mastenbroek is a one-woman band. What started off as a job in retail 30 years ago turned into sniffing out every nuance between hundreds of ingredients and concocting her own brand. As well as creating products for industry giants including Jo Malone, Jigsaw and David Austin Roses, she is a chemist, evaluator, salesman and past president of the British Society of Perfumers.

"I read chemistry at Oxford, but I wasn't a straight kind of chemist. I liked the idea of creating something beautiful, I had an artistic flair but I didn't know anything about perfumery."

As an assistant sales manager at Selfridges in the perfumery department, Mastenbroek went on an evening course to discover how perfume is made, which revealed "this wonderful new world".

"I applied to several perfumery companies and got a job with Naarden International [a Dutch-English company, now Givaudan] to train as a perfumer. My training started in England, then I had to take a smelling test, as a result of which I started more structured training at the Naarden head office in Holland."

Between Holland and moving back to the UK, Mastenbroek's husband's job took them to Japan where she worked in Naarden's creative development unit in Yokohama. Ten years ago she went solo and set up Fragosmic Ltd. She gets her ingredients delivered to her lab at home in Beaconsfield, Buckinghamshire, from a perfumery manufacturers in Wellingborough with whom she has had a long standing working relationship with.

At the outset, perfume making seems a simple case of mixing and matching, but it's more of a balancing act.

"When you build a perfume, you create a structure around three notes," Mastenbroek says. "You get a top, middle and a base note. Creating a distinctive smell is down to what kind of ratio you want in a perfume. If the base is overpowering it's going to swallow up the other combinations."

The initial hit from a spray is the top note working its magic. It acts as an attention grabber until it diffuses and lets other notes pervade. "Creating a perfume for a client takes six to eight weeks after listening to what they want and modifying it accordingly. However my signature scent, released in 2010, took three years. It is essential to let the scents macerate and develop among themselves."

You won't need to cite the periodic table, but an understanding of chemistry is important to decipher the properties and reactivity of ingredients when making a fragrance. "My chemistry still gives me a greater understanding because if something is an aldehyde [an organic compound] it tells me how reactive it is." But the alchemy of perfume making goes beyond science.

Moments from Mastenbroek's childhood waft out of her second fragrance, Amorosa. "Watermelon featured in my American childhood. When I would go down to the beach in New Jersey there would be stands on the side of the road where watermelons were sold and the smell was amazing," she recalls.

For a job lead by the nose, a perfumer is always at work. "Last week I was raking fallen apples and I smelt something that reminded me of an old, rusty, mossy smell. Often I smell something and then I'll want to use it."

But for such a crowded market the key is to think outside the bottle: "I welcome modern experimenting with unusual scents. There has been a proliferation of niche branding and each one of them is finding his own market."

Perfumery is a small industry – supposedly there are more astronauts than there are perfumers in the world– but the BSP is a good place for aspiring perfumers to start, although there other schools across the UK. Tenacity is crucial. "You have to be incredibly curious about the ingredients and dedicated to learn to differentiate between the smells," Mastenbroek says.